The production and use of cellulose food casings for the manufacture of frankfurters and the like is well known in the art. Automatic stuffing machines are available which stuff an uncooked meat or poultry emulsion into a long tubular casing while simultaneously forming the stuffed casing into links. The result is a long string of sausage links Lip to 40 to 50 meters long or longer.
The long string is processed to cook or cure the emulsion. This often involves exposing the string to a natural smoking operation or to a shower of liquid smoke. In either case the smoke permeates the casing and imparts both color and taste to the encased sausage. After processing, the casing is removed in a high speed peeling operation to produce individual sausages ready for retail packaging. Typically, "skinless" frankfurters are produced in this fashion.
At present, the only commercial process for manufacturing a cellulose casing involves the well-known viscose process. In the viscose process, a natural cellulose is contacted with a strong base to produce alkali cellulose. The alkali cellulose then is reacted with other chemicals to produce cellulose xanthate, a soluble cellulose derivative. The xanthate is dissolved in an aqueous solution of sodium hydroxide and extruded as a tube into an acid bath. The acid reacts with the xanthate to regenerate the cellulose in the form of a hydrated cellulose gel. Thus, with the viscose process, there is a first chemical reaction to create a soluble cellulose derivative and a second chemical reaction to regenerate the cellulose from the derivative. The gel is then washed, plasticized with a polyol such as glycerin and then is dried from a moisture content of 200% or more to a moisture content of less than 15% and preferably to about 5% to 10% based on the weight of dry cellulose in the casing or "bone dry gauge" (BDG).
Drying sets the properties of the tubular cellulose casing. Typically the dry casing then is passed through a steam chamber to remoisturize the cellulose to a moisture level of about 10 to 25% BDG. At this level the casing is sufficiently pliable to permit further handling without damage.
More recently, a solution process has been adapted to the production of cellulose casing. Reference is made to U.S. Pat. Nos. 5,277,857 and 5,451,364 for details of the process. In brief, in the solution process the natural cellulose undergoes a direct dissolution by a cellulose solvent such as N-methyl-morpholine-N-oxide (NMMO). The resulting cellulose solution is thermoplastic in that it is solid at room temperature. The solution is extruded at about 100.degree. C. as a tube into a bath containing a non solvent for the cellulose such as water. In the bath the solvent is extracted from the extruded tube to precipitate or regenerate the cellulose as a hydrated cellulose gel. Thus, in the solution process there is no chemical reaction and the cellulose is non derivitized. For purposes of the present invention, "non derivitized" cellulose means a cellulose which has not been subjected to covalent bonding with a solvent or reagent but which has been dissolved by association with a solvent or reagent through Van der Waals forces and/or hydrogen bonding. As in the viscose process, the tube of cellulose gel is washed (to remove residual solvent), dried to form a cellulose film and set properties and then the dried film is remoisturized.
As noted above, after the casing is stuffed and processed to form a string of sausage links, the casing is peeled from the sausages in a high speed peeling operation. One problem associated with the high speed peeling is that the process is not always 100% successful and occasionally, bits or sections of casing remain on the sausage. Since the cellulose casings are transparent, the presence of casing on the sausage is difficult to determine by visual inspection.
To facilitate detection by visual inspection, it is customary to provide the transparent casing with indicia, such as stripes or to color the casing. As used hereinbelow, the term "indicia" unless otherwise indicated, is intended to mean any color, logo, stripe or other marking either to facilitate detection of casing on the surface of a peeled sausage or to function as an identifier for other purposes.
It is common to expose the string of sausage links to a smoke treatment. Accordingly, it is important that the indicia on the casing not interfere with the ability of the smoke to permeate the casing. Otherwise, the indicia will act as a mask that prevents the passage of the smoke so that the resulting sausage will be lighter in color below the indicia. It also is important that the ink or color comprising the indicia does not rub off during the handling of the casing or the string of sausages.
Also, it is customary to print logos or other identifying information on casing. Stripes or color to facilitate casing detection after stuffing can be applied in this fashion. However, printing off line adds handling and manufacturing steps to the process. Accordingly, for purposes of economy, it is desirable to apply the indicia in-line with the casing manufacturing process.
EP 0 738 471 discloses a casing for use in so called "shadow printing" which takes advantage of the printing being a barrier to the passage of smoke. Here, the casing is printed with a liquid polyvinylidene chloride (PVDC) containing a pigment. The printed area acts as a mask so that after the sausage is smoked and the casing is peeled off, the surface of the sausage beneath the printing is a lighter color. In this manner the image of the printing is, in effect, transferred to the sausage. The '471 Patent gives no details of the condition of the casing when the printing is applied other than to say that various printing methods such as ink jet, flexography or mezzotint can be used to apply the PVDC to the casing. However, since flexography and mezzotint are suggested printing methods for the PVDC ink it is believed that the printing as described in the '471 Patent is applied to the casing in an off line operation.
U.S. Pat. No. 2,860,052 discloses an attempt to put a contrasting color pattern on a casing in-line with the manufacturing process. In U.S. Pat. No. 2,860,052 a naphthalate dye is applied by a print roll that bears directly against the surface of a flattened gel casing as the casing leaves a wash tank. The casing then is passed through a color salt where the coupling of the dye and salt produce the desired color pattern on the surface of the casing. It is not believed that the process as disclosed in this U.S. Patent is commercial. Also, bearing a print roll directly against the gel casing is risky in that the gel casing is easily torn and pressing against it may compromise the final properties of the cellulose film.
The current commercial practice for providing a contrasting mark or color on the casing in-line with the casing manufacturing process is represented by U.S. Pat. No. 3,334,168 and Canadian Patent No. 2,047,477. In both patents the color is incorporated into the extruded viscose such that the casing produced can be more easily detected on the peeled sausages.
U.S. Pat. 3,334,168 discloses mixing a pigment, such as carbon black, with viscose and injecting a stream of the mixture into the wall of the extruded tube of viscose. Upon regeneration of the cellulose, the injected stream appears as stripes imbedded in the wall of the casing.
Canadian Patent No. 2,047,477 discloses dispersing a water soluble pigment uniformly throughout the viscose except for an area running longitudinally along the extrusion so that after regeneration of the viscose, the casing wall is uniformly colored except for a clear window running the length of the casing. Neither the injection of a carbon black pigment nor the coloring of the viscose with a soluble dye act as barriers to the diffusion of smoke through the casing.
While the in line methods disclosed in U.S. Pat. 3,334,168 and Canadian 2,047,477 are suitable for a derivitized casing, they are not suitable for non derivitized cellulose casing. This is because the addition of a dye or pigment into the stream of the molten cellulose solution possibly can alter the properties of the resulting casing. The dye or pigment added to the cellulose solution also may compromise the subsequent recovery of the solvent which is important to the economics of making a non derivitized cellulose casing. Accordingly, when extruding the non derivitized cellulose casing, it is preferred not to introduce a foreign element such as a dye or pigment into the solution stream. This means that any color or indicia must be applied after the solvent has been removed and the hydrated cellulose gel is formed.
When applying Indicia to the surface of a cellulose casing in an off line process, it is conventional to incorporate a dye or pigment into a carrier which cross links to form a film and which cross links with the cellulose substrate or bonds to the cellulose substrate such as by hydrogen bonding. This cross linking prevents the indicia from rubbing off when the casing is rewetted as occurs during stuffing. However, forming a film which cross links with the cellulose presents other problems for the stuffing operation. For example, a film applied to the casing surface can act as a mask and prevent uniform smoke penetration. Accordingly, the problem is to apply a stripping or other identifiable indicia to the casing which does not mask the passage of smoke.
Thus, the desiderata of the present invention include applying indicia to either a derivitized or non derivitized cellulose casing in an in-line process that does not bring any printing means to bear against the casing and that provides indicia which adheres to the casing surface by cross linking without masking the passage of smoke.
Accordingly, an object of the method of the present invention is to apply indicia to the exterior of a cellulose casing in-line with the casing manufacturing process.
Another object of the method of the present invention is to apply indicia to the exterior of the casing in a non contact printing operation.
A further object of the method of present invention is to apply indicia to the exterior of a cellulose casing in the form of an ink including a carrier which is crossed linked to form a film and which cross links to the cellulose to bind the film to the cellulose yet does not mask the passage of smoke, the cross linking being initiated by heat.
Yet another object of the present invention is a cellulose food casing having indicia which is cross linked to the casing surface and yet permits the uniform diffusion of smoke through the casing.